Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 210314

Area Forecast Discussion
National Weather Service Seattle WA
815 PM PDT Tue Mar 20 2018

.SYNOPSIS...Moist southwest flow will develop Wednesday, and
a cold front will move inland early Thursday. A deep upper level
low will make the air mass cool and unstable Thursday through
Saturday. The snow level will fall to around 1000 feet Friday and
Saturday, and isolated thunderstorms are possible Friday.


.SHORT TERM...Mostly fair skies and light winds tonight with
low temps mainly in the 30s. Wet weather will return Wednesday and
Thursday as a deep upper level low forms over the NE Pacific,
setting up moist S/SW flow over WA. A cold front will shift inland
Thursday morning, the focus for steadier precip. This timing is
slightly slower compared to previous solutions. A trough axis will
then swing through Thursday afternoon keeping the air mass cool and
unsettled for additional showers. Temperatures will trend below
normal on Thursday with highs in the 40s. Lows will be in the 30s
Thursday night. The mountains will see a few inches of snow with
snow levels lowering to around 1500 feet.

The broad upper level low will remain over the region on Friday
for more cool and showery weather. We are still under moist,
diffluent flow and there`s a slight chance of thunderstorms across
much of Western WA. By this point in time, 500 mb temps will drop
to around -38 C, setting up steep lapse rates and a Lifted Index
near 0 and showers with some graupel.

.LONG TERM...The deep upper level low will finally weaken and
shift inland as an open trough on Saturday. Snow levels will
remain low during the morning, only a few hundred feet, but
showers in the lowlands are light and spotty. Significant snowfall
accumulations are not expected. Temperatures will continue to
trend below normal with highs mainly in the 40s. Models are now a
little wetter on Sunday as an upper level short wave trough clips
the area. PoPs were increased accordingly. A warm front will
reach the area on Tuesday for more wet weather. The air mass will
moderate for slightly warmer conditions and rising snow levels.


.AVIATION...The air is generally dry and stable except for a few
patches of clouds--one near KCLM that might finally be breaking up
this evening. Elsewhere, there are some high clouds, otherwise fair
skies. Moisture over Oregon will move north into the area starting
around daybreak Wed and areas of rain should develop by evening as
the southerly flow aloft continues to draw moisture up into the area.

KSEA...Fair skies tonight and then mid level clouds increase
Wednesday as moisture pushes north from Oregon. Rain should develop
later in the day. Some periods of rain should arrive Wednesday night.


.MARINE...Weak high pressure will give way to a low pressure system
that will drop down over the region Wednesday. Stronger southerly
winds are likely to develop starting on Thursday, with small craft
advisory winds possible over portions of the area waters into the

.HYDROLOGY...River flooding is not expected in the next 7 days.



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